p(t)  = p_i + \frac{\Delta Q(t)}{V_\phi \cdot c_t}

where

	Cumulative Voidage Replacement Balance (CVRB) over time
	initial drainage volume of the main pay (excluding the aquifer and gas cap)
	initial porosity
	total compressibility
	pore compressibility
	initial water saturation
	initial gas saturation
	initial oil saturation:
	fluid compressibility of water phase, oil phase and gas phase

The MatBal equation can be re-written as following:

p = p_i + \frac{\delta Q}{c_\phi \, V_\phi} + \delta p_i

\delta p_i =  \frac{ B_{og} \, F_{Oi} + B_{go} \, F_{Gi} + B_w \, F_W -1}{c_\phi}

B_{og} = \frac{B_o - R_s \, B_g}{1- R_s \, R_v}

B_{go} = \frac{ B_g - R_v \, B_o}{1- R_s \, R_v}

where

See Also